Surfaces are heated either to achieve a uniform temperature distribution, such as for heated rolls in paper making, or to achieve areas of high and low temperature, such as for the surface of a cooktop.
For achieving uniform temperatures throughout a region of the surface, steam, or another hot fluid, is often circulated through channels cut under the surface. Alternatively, electrical resistance heaters may be inserted below the surface of a structure, where the structure is required to be designed for high thermal conductivity. In yet other designs, radiant heaters are configured to illuminate the heating surface uniformly from above or below. Often, the heated surfaces are supported by massive substrates for storing heat. The result is often a surface that exhibits some degree of temperature uniformity but with poor or slow temperature control, especially when there are variable thermal loads, rapid heating or cooling process conditions, or geometric discontinuities in the heated region of the surface, such as corners and edges.
In the example of a cooktop, where areas of high temperature are needed with adjacent areas of low temperature, large, discrete gas burners or electrical resistance elements are distributed over the surface to provide specific locations where independent temperature control is available for heating generally a small number of cooking utensils. In other systems, electric or gas heating elements are embedded in or under cooking surfaces that conduct heat laterally to a greater or lesser extent. The limitations of these systems typically are the small number of fixed locations on the surface where high temperatures are achievable, the fixed size of areas that can be heated, poor thermal efficiency, and no provision for indicating that an area of the surface is still hot after power is cut.
A second example of a heated surface with variable temperatures is a thermal print head. Here, an array of up to six-hundred (600) minute resistors dispense a tiny quantity of energy into an ink channel to form a bubble that creates a jet of fluid. Each resistor is addressable and is controlled independently from the others. A limitation of the thermal print head is size and power.
There is a clear need, therefore, for a more active surface for temperature control, whereby the surface can achieve accurate, uniform temperatures when desired, regardless of location on the surface, part geometry, process heating conditions, or thermal load. In addition, there is a clear need for surfaces that can respond to multiple demands for high, differing temperatures at arbitrary areas without unduly heating adjacent areas, while providing a visual indication of temperature for each arbitrary area.